Brain function is based upon the precise connectivity of a large number of neurons. Connectivity in turn depends in large part on the genetically determined wiring properties of neurons, including their neurite projection, branching, and placement of synaptic contacts with specific partners. To understand and manipulate brain circuits one needs a detailed knowledge of how the genes expressed in a developing neuron control the wiring properties of this cell. For genetic studies, Drosophila offers many advantages, in that virtually every gene can be targeted for knock-out or activation in a cell type selective manner. More importantly in the context of studying neuronal circuitry, the Drosophila brain is composed of a manageable number of stereotyped neuronal lineages, groups of neurons descended from individual stem cells (neuroblasts) born in the embryo. During the course of its proliferation, each neuroblast expresses characteristic sets of genes (transcription factors) which are thought to specify the wiring properties of the neurons born from that particular neuroblast during a particular time interval. These neurons form a so called sublineage. To learn about the genetic control of brain circuitry we and others have taken the approach to document the structural properties of lineages and sublineages, and correlate them to the dynamic pattern of gene expression in the neuroblast. During the previous funding period we have generated detailed maps and 3D models of all lineages constituting the adult and larval brain. We here propose three aims that continue and extend this work. First, we will reconstruct the connectivity of a subset of larval brain lineages and their sublineages that form a particular, well characterized circuit. This reconstruction will be done at a so far unparalleled level of resolution, using a series of several thousand contiguous electron microscopic sections in conjunction with a specially developed software package that allows us to assign all synapses to specific neurons and their lineages. Secondly, we will link the structurally defined lineages mapped in the larval brain with the neuroblasts of the embryo, using a technique that systematically labels all transcription factors expressed in neuroblasts and then follows the expression of these genes from neuroblast to lineage. Thirdly, we will screen for and genetically characterize genes that play a role in directing lineages to their proper place in a circuit.